Are you sure you are only reading the reflection or echo from the wall that you have? How about reflections ? And if they are not filtered out somehow, I am not sure if you can get a repeatable reading. The direction sensitivity of the reciever section would be a place to check.

Note: The above is just a solution that came to mind. Maybe all Ultrasonic recievers are highly direction sensitive - I would'nt know.

Raghu